The present invention relates to a polyester multilayered film for high-density magnetic discs. More particularly, the present invention relates to a polyester multilayered film which has excellent surface properties, and is lowered in falling-off of particles in use and also high in scratch durability.
Magnetic discs, especially floppy discs are being more and more densified recently. The floppy discs having a memory capacity of 4 MB are now commercially available, and attempts are being made for realizing a floppy disc whose the memory capacity exceeds 10 MB. Accompanied with such densification of floppy discs, the request for the improvement of the properties of a base film for floppy discs has become strong - especially the request for the improvement of the surface properties of the base film is most exacting.
In the currently available base films for high-density floppy discs, there are mostly added to the base films the TiO.sub.2 particles having characteristics which lower the formation of coarse protuberances while increasing the protuberance density in the film surface. By adding TiO.sub.2 particles, the surface properties of the film can be indeed improved, but such film has the problem that the added particles tend to fall off and the fell-off particles would become a cause of drop out, resulting in a reduced productivity of floppy discs. It has been also pointed out that the film using the TiO.sub.2 particles is susceptible to fine scratches, and this has become a serious problem with raising of disc density.
As a result of earnest studies for dissolving the said problems, it has been found that by laminating a layer of polyester (B) on at least one side of a layer of polyester (A) by the coextrusion method so as to have surface properties represented by the formulae (1) to (3) and the thickness of not more than 3 .mu.m, the obtained polyester multilayered film has excellent surface properties as well as high scratch durability and abrasion resistance, and is useful as base film for high-density magnetic discs: EQU 0.002.ltoreq.Ra.ltoreq.0.015 (1) EQU F.sub.3 =0 (2) EQU 50.ltoreq.Pc (3)
[wherein the Ra is a center line average roughness (.mu.m) of the surface of the layer of polyester (B); the F.sub.3 is the number of the protuberances having a height of 0.81 .mu.m or greater, counted per 25 cm.sup.2 in the surface of the layer of polyester (B); and the Pc is a peak count (number of peaks per 0.8 mm) in the surface of the layer of polyester (B)].
The present invention was achieved on the basis of the above finding.